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Abstract:

A semiconductor device in one embodiment has a first connection region, a
second connection region and a semiconductor volume arranged between the
first and second connection regions. Provision is made, within the
semiconductor volume, in the vicinity of the second connection region, of
a field stop zone for spatially delimiting a space charge zone that can
be formed in the semiconductor volume, and of an anode region adjoining
the first connection region. The dopant concentration profile within the
semiconductor volume is configured such that the integral of the ionized
dopant charge over the semiconductor volume, proceeding from an interface
of the anode region which faces the second connection region, in the
direction of the second connection region, reaches a quantity of charge
corresponding to the breakdown charge of the semiconductor device only
near the interface of the field stop zone which faces the second
connection region.

Claims:

1. A method for fabricating a field stop zone within a semiconductor
device, the method comprising: providing a semiconductor device
comprising a first and second connection region, and a semiconductor
volume arranged between the first and second connection regions;
performing a plurality of proton irradiations on the semiconductor
volume, each of the plurality of proton irradiations comprising an
implantation energy and a proton dose and at least one proton irradiation
comprising an implantation energy, the plurality of proton irradiations
providing at least first penetration depth and a second penetration
depth, the first and second penetration depths being unequal; and
performing at least one heat treatment on the semiconductor volume, the
at least one heat treatment comprising a temperature.

2. The method of claim 1, wherein the semiconductor device is an IGBT
device; and wherein a highest implantation energy used is approximately
at least 1 MeV, and a lowest implantation energy used is approximately at
most 500 keV.

3. The method of claim 1, wherein the performing a plurality of proton
irradiations step further comprises: performing three proton irradiations
on the semiconductor volume at approximately the following implantation
energies: 300 keV, 500 keV, and 1 MeV.

4. The method of claim 1, wherein the performing a plurality of proton
irradiations step further comprises: performing four proton irradiations
on the semiconductor volume at approximately the following implantation
energies: 300 keV, 500 keV, 1 MeV and 1.25 MeV.

5. The method of claim 1, wherein a lowest implantation energy used is
approximately at least 500 keV.

6. The method of claim 1, wherein the performing a plurality of proton
irradiations step further comprises: performing three proton irradiations
on the semiconductor volume in the following ranges for the implantation
energies respectively: between 500 keV and 700 keV, between 1300 keV and
1700 keV, and between 1800 keV and 2400 keV.

7. The method of claim 1, wherein the performing a plurality of proton
irradiations step further comprises: performing four proton irradiations
on the semiconductor volume in the following ranges for the implantation
energies respectively: between 300 keV and 500 keV, between 500 keV and
900 keV, between 900 keV and 1300 keV, and between 1200 keV and 1700 keV.

8. The method of claim 1, wherein the implantation energy of at least one
proton irradiation is above 2 MeV and the temperature of at least one
heat treatment step is equal or above 400.degree. C.

9. The method of claim 1, wherein the performing at least one heat
treatment step further comprises: performing the at least one heat
treatment at a temperature of approximately 350 to 550.degree. C., the at
least one heat treatment being carried out between proton irradiations or
after the proton irradiations.

10. The method of claim 1, wherein the performing a plurality of proton
irradiations on the semiconductor volume step, further comprises:
performing at least one proton irradiation at an irradiation dose of
approximately 1*10.sup.13 protons/cm2 to produce doping regions
situated deeper in the semiconductor volume; and performing at least one
proton irradiation at an irradiation dose of approximately 7*10.sup.13
protons/cm2 to produce doping regions situated nearer to a surface
of the semiconductor volume.

11. The method of claim 1, wherein the semiconductor device comprises a
diode, and wherein the irradiation doses of the plurality of proton
irradiations are between approximately 0.5*10.sup.13 to 20*10.sup.13
protons/cm.sup.2.

12. The method of claim 1, wherein the semiconductor device comprises a
diode, and further wherein a sum of the irradiation doses of the
plurality of proton irradiations is between approximately 2*10.sup.13
protons/cm2 to 50*10.sup.13 protons/cm.sup.2.

13. The method of claim 1, wherein the plurality of proton irradiations
further comprising a deepest penetration depth and a second deepest
penetration depth, the second deepest penetration depth being
approximately 30% to 60% of the value of the deepest penetration depth.

14. The method of claim 1, further comprising: selecting the implantation
energy and the proton dose of each proton irradiation of the plurality of
proton irradiations and the temperature of the at least one heat
treatment step so as to produce a dopant concentration profile such that
the dopant concentration profile includes one maxima within the field
stop zone.

15. The method of claim 1, wherein a sum of the electrically active
dopant dose created by the plurality of proton irradiations is between
approximately 3*10.sup.11 donors/cm2 to 1*10.sup.12 donors/cm.sup.2.

16. The method of claim 1, wherein the plurality of proton irradiations
further comprise a deepest penetration depth of at least 6 μm.

17. The method of claim 1, further comprising: selecting the implantation
energy and the proton dose of each proton irradiation of the plurality of
proton irradiations and the temperature of the at least one heat
treatment step so as to produce a dopant concentration profile such that
in a reversed biased mode of the semiconductor device the gradient of the
electric field within the field stop zone is larger than the gradient of
the electric field within the drift zone.

18. The method of claim 1, wherein the proton dose of a proton
irradiation is lower for a higher implantation energy.

19. The method of claim 1, wherein at least one heat treatment is
performed by laser annealing.

20. The method of claim 1, further comprising forming a metal layer on a
surface of the semiconductor device, wherein at least one heat treatment
is performed before the forming of the metal layer and after at least one
proton irradiation.

21. The method of claim 1, wherein the thickness of the field stop zone
is at least 10% of the thickness of the semiconductor volume arranged
between the first and second connection regions.

22. The method of claim 1, wherein the distance of two adjacent
penetration depths of two proton irradiations is at least 5 μm.

23. The method of claim 1, further comprising selecting the implantation
energy and the proton dose of each proton irradiation of the plurality of
proton irradiations and the temperature of the at least one heat
treatment step so as to produce a dopant concentration profile such that
an integral of an ionized dopant charge over the semiconductor volume,
proceeding from a pn junction provided between the first connection
region and a field stop zone, in the direction of the second connection
region, substantially reaches a quantity of charge corresponding to a
breakdown charge of the semiconductor device only near an interface of
the field stop zone that is closest to the second connection region.

Description:

RELATED APPLICATIONS

[0001] This application is a Continuation Application of application Ser.
No. 13/186,470, which was filed on Jul. 20, 2011. Application Ser. No.
13/186,470 is a Continuation Application of application Ser. No.
12/416,935, which was filed on Apr. 2, 2009. Application Ser. No.
12/416,935 is a Continuation Application of Ser. No. 11/241,866, which
was filed on Sep. 30, 2005 and which claimed benefit of German Patent
Application No. 10 2004 047 749.3, which was filed on Sep. 30, 2004. The
priority and entire contents of application Ser. Nos. 13/186,470,
12/416,935 and 11/241,866 and German Patent Application No. 10 2004 047
749.3 are hereby claimed and incorporated herein by reference.

BACKGROUND

[0002] The invention relates to a semiconductor device and to a
fabrication method suitable therefore.

[0003] If semiconductor devices are intended to have a soft switching
behaviour, they must be designed in such a way as to avoid current
chopping during switching. Current chopping occurs for example during
hard commutation of diodes. The consequence of such current chopping is
that severe voltage or current oscillations occur. If such oscillations
exceed maximum values permissible for the diode, then destruction of the
diode may occur. Destruction of the diode may also be caused by excessive
interference effects on driving processes which are brought about by the
current or voltage fluctuations, and resultant incorrect behaviour of the
driving processes. The problem area described above occurs particularly
in the case of circuits having high leakage inductance, high currents
(for example in the case of power semiconductors being connected in
parallel to a great extent) and at high voltages with respect to which
the diode is commutated.

[0004] In order to realize diodes having a soft switching behaviour, the
thickness of the diodes has been designed such that at maximum voltage
the space charge zone that forms, proceeding from the pn junction formed
by the p-doped anode region and the adjoining lightly n-doped base region
in the semiconductor volume, does not reach the highly n-doped rear-side
emitter. However, this entails high on-state losses and switching losses,
since the overall losses of semiconductor devices, in particular bipolar
semiconductor devices, increase approximately quadratically with the
thickness of the lightly doped base region (chip thickness). A soft
switching behaviour is difficult to realize particularly for high-voltage
devices (having a rated voltage of more than 150 V, in particular
starting from a rated voltage of approximately 500 V), since a basic
material with a doping concentration that is significantly lower than
would be necessary for achieving the required reverse voltage is usually
used for fabricating such components. The low doping concentration of the
basic material serves for realizing the DC voltage blocking stability of
the semiconductor device, which in turn necessitates sufficiently low
field strengths at the anode and in the region of the edge termination of
the semiconductor device. The low basic doping has the effect that the
space charge zone propagates very far, which has to be compensated for by
means of a large chip thickness of the semiconductor device if the
intention is to ensure that the space charge zone does not reach the
rear-side emitter.

[0005] In order to keep down the chip thicknesses, it has been proposed to
introduce a field stop zone, that is to say a zone of increased doping,
in the semiconductor volume of the semiconductor device, which zone may
be configured in stepped fashion, for example. FIG. 1 shows a
corresponding doping profile 1 with a stepped field stop zone using the
example of a diode. What is disadvantageous in this case is that
difficult and expensive processes are required for producing the stepped,
inhomogeneous doping profile 1: thus, an epitaxial method is required for
example for fabricating the doping profile (the high doping of the
carrier substrate on which the epitaxial layer is deposited is not
illustrated in FIG. 1). As an alternative, it is possible to use a
diffusion process, but this would take up about 100 hours at a process
temperature of 1200° C. and so is not very suitable in practice. A
doping profile 2 that can be produced by means of such a diffusion
process is likewise indicated in FIG. 1.

SUMMARY

[0006] The semiconductor device has a first connection region, a second
connection region, and a semiconductor volume arranged between the first
and second connection regions, there being provided within the
semiconductor volume, in the vicinity of the second connection region, a
field stop zone for spatially delimiting a space charge zone that can be
formed in the semiconductor volume. The dopant concentration profile
within the semiconductor volume is configured such that the integral of
the ionized dopant charge over the semiconductor volume, proceeding from
a pn junction provided between the first connection region and the field
stop zone, in the direction of the second connection region, reaches a
quantity of charge corresponding to the breakdown charge of the
semiconductor device only near the interface of the field stop zone which
faces the second connection region, the pn junction being the last pn
junction before the field stop zone, relative to a direction pointing
from the first connection region to the second connection region.

[0007] The semiconductor device according to the invention can be embodied
in particular as a diode or as an IGBT (insulated gate bipolar
transistor). A diode according to the invention has a first connection
region, a second connection region, and a semiconductor volume arranged
between the first and second connection regions, there being provided
within the semiconductor volume, in the vicinity of the second connection
region, a field stop zone for spatially delimiting a space charge zone
that can be formed in the semiconductor volume, and an anode region
adjoining the first connection region. The dopant concentration profile
within the semiconductor volume is configured such that the integral of
the ionized dopant charge over the semiconductor volume, proceeding from
an interface of the anode region which faces the second connection
region, in the direction of the second connection region, reaches a
quantity of charge corresponding to the breakdown charge of the
semiconductor device only near the interface of the field stop zone which
faces the second connection region.

[0008] The IGBT according to the invention has a first connection region,
a second connection region, and a semiconductor volume arranged between
the first and second connection regions, there being provided within the
semiconductor volume, in the vicinity of the second connection region, a
field stop zone for spatially delimiting a space charge zone that can be
formed in the semiconductor volume, and a body region adjoining the first
connection region. The dopant concentration profile within the
semiconductor volume is selected such that the integral of the ionized
dopant charge over the semiconductor volume, proceeding from an interface
of the body region which faces the second connection region, in the
direction of the second connection region, reaches a quantity of charge
corresponding to the breakdown charge of the semiconductor device only
near the interface of the field stop zone which faces the second
connection region.

[0009] If this condition is met, then the space charge zone reaches far
into the semiconductor volume when a reverse voltage is present, but is
increasingly curbed as the reverse voltage rises. The spatial utilization
of the semiconductor volume by the space charge zone is thus optimized.
Furthermore, by meeting the conditions mentioned above it is guaranteed
that during the switching operation, the increase in the reverse voltage
is always associated with the depletion of a charge packet of flooding
charge present. This prevents an abrupt rise in the voltage during the
switching of the semiconductor device and thus guarantees a soft
switching behavior.

[0010] The thickness of the field stop zone, which is generally configured
in layered fashion, should be more than 5%, preferably between 20% and
40%, of the thickness of the semiconductor volume, that is to say that
the profile of the field stop zone should be configured in a manner
leading out deeply (reach deep into the semiconductor volume). In a
preferred embodiment, the dopant concentration profile is designed as a
curved profile with a plurality of maxima (peaks), in which case the
height of the peaks should increase, or at least not decrease
significantly in the direction toward the second connection region.

[0011] The field stop layer may directly adjoin the second connection
region, or else be spaced apart from the latter.

[0012] Preferably, the thickness of the field stop zone is a maximum of
one third of the base width of the semiconductor volume, the base width
being defined as the distance between the last pn junction before the
field stop zone and the interface of the field stop zone which faces the
second connection region.

[0013] If the semiconductor device is configured as a diode, then a
cathode region adjoining the second connection region is formed within
the semiconductor body. If the semiconductor device is designed as a
diode, the doping concentration within the field stop zone is preferably
10 to 30 times the basic doping of the semiconductor volume. Furthermore,
the breakdown charge for typical basic dopings is approximately
1.8*1012 doping atoms/cm2.

[0014] If the semiconductor device according to the invention is
configured as an IGBT, then a rear-side emitter adjoining the second
connection region is formed within the semiconductor device. The field
stop layer may directly adjoin the rear-side emitter, or else be spaced
apart from the latter.

[0015] The invention can be applied, in principle, to all semiconductor
devices having a field stop zone, e.g. bipolar transistors, GTOs (gate
turn-off), MOS transistors, etc.

[0016] The invention furthermore provides a method for fabricating a field
stop zone within a semiconductor device according to the invention. In
this method, the semiconductor volume is exposed to a plurality of proton
irradiations and at least one heat treatment step, the acceleration
energies and proton doses of the respective proton irradiations and also
the temperature of the heat treatment step or of the heat treatment steps
being chosen so as to produce the required dopant concentration profile.

[0017] Preferably, radiation is effected through the second connection
region (rear-side), that is to say--in the case of a semiconductor device
with a vertical construction--through the rear-side of the semiconductor
device. In principle, it is also possible to radiate through the top side
of the semiconductor device, that is to say through the first connection
region. However, higher irradiation energies would be necessary in this
case.

[0018] If the semiconductor device is designed as an IGBT device, in a
preferred embodiment the highest energy of the protons used during the
implantation is at least 1 MeV, and the lowest implantation energy used
is a maximum of 500 keV. It is thus possible, for example, to perform
three proton irradiations of the semiconductor volume which have the
following implantation energies: 300 keV, 500 keV and 1 MeV. As an
alternative four proton irradiations of the semiconductor volume may be
performed, the corresponding energy doses being 300 keV, 500 keV, 1 MeV
and 1.25 MeV.

[0019] Heat treatment processes that are effected at a temperature of 350
to 420° C. are carried out between the proton irradiations or
after proton irradiations. As an alternative, heat treatment processes at
temperatures of 420 to 550° C. may be effected between or after
the proton irradiations.

[0020] Preferably, an irradiation dose of approximately 1*1013
protons/cm2 is chosen for the purpose of producing doping regions
situated deeper in the semiconductor volume (large distance from the
second connection region), while an irradiation dose of approximately
5*1013 protons/cm2 is chosen for the purpose of producing
doping regions situated nearer to the surface of the semiconductor volume
(small distance with respect to the second connection region). In this
case, the sum of all the irradiation doses is intended to be 5*1013
protons/cm2 to 50*1013 protons/cm2.

[0021] If the semiconductor device is intended to be designed as a diode,
then in a preferred embodiment the irradiation doses of the individual
proton irradiations are 0.5*1013 to 20*1013 protons/cm2.
The sum of all the irradiation doses of the individual proton
irradiations should be 5*1013 protons/cm2 to 50*1013
protons/cm2. Heat treatment processes that are effected at a
temperature of 350 to 550° C. may be carried out between the
proton irradiations or after the proton irradiations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention is explained in more detail below in exemplary
embodiment with reference to the figures, in which:

[0023]FIG. 1 shows the dopant concentration profile in the case of a
diode with a rated reverse voltage of 1200 V with a field stop zone
(punch-through embodiment).

[0024]FIG. 2 shows an example of a dopant concentration profile of a
field stop layer of a semiconductor device according to the invention.

[0025] FIGS. 3a-3c show the switching behavior of a semiconductor device
according to the invention in comparison with a reference device using
the example of a diode.

[0026]FIG. 4 shows a schematic diagram of the method for fabricating a
diode according to the invention.

[0027]FIG. 5 shows a schematic diagram of the method for fabricating an
IGBT according to the invention.

DESCRIPTION

[0028] The exemplary application of a diode shall be discussed first.

[0029] The method according to the invention for fabricating the field
stop zone in this case provides for simulating the stepped field stop
zone doping profile 1 known in principle from FIG. 1 by the doping effect
of one or more proton irradiations with at least one subsequent heat
treatment step or heat treatment steps taking place between the
irradiations. One advantage is that in the case of irradiation with
protons, it is possible to achieve relatively large depths with lower
implantation energies than in the case of conventional dopants. The
doping is effected predominantly in the so-called end-of-range region of
the implantation, and to a lesser extent in the region radiated through.
In the case of an implantation from the rear-side, it is possible, by way
of example, with an acceleration voltage of 1.5 MeV, to effect
implantation to a depth of almost 30 μm in silicon. By varying the
irradiation energy and dose, it is thus possible to produce virtually
arbitrary, rising, falling, constant or else dopant concentrations which
have one or a plurality of minima or maxima. Moreover, activation of a
proton doping merely requires a heat treatment step at 350° C. . .
. 550° C., whereas conventional dopings have to be annealed at
more than 800° C.

[0030] It is particularly advantageous if the concentration in the field
stop zone is chosen not to be too high (e.g. maximum factor of 10 to 30
times higher than the basic doping) and, up to shortly before the
rear-side emitter, the integral dopant dose comprising homogeneous basic
doping and field stop just reaches the breakdown charge of approximately
1.8*1012 dopant atoms/cm2. The integral dopant dose is intended
to exceed the breakdown charge only directly before and at the rear-side
emitter. By virtue of this choice of the dopant concentrations or the
integral dopant profile, the space charge zone reaches into the
semiconductor as far as possible, but in increasingly curbed fashion as
the reverse voltage rises. It is thus always necessary for flooding
charge to be depleted. As long as an increase in the reverse voltage by
DELTA U is necessarily associated with the depletion of a charge packet
DELTA Q of the flooding charge, that is to say the depletion is not
chopped off, the reverse voltage cannot rise abruptly: the switching
profile remains "soft". By virtue of the doping profile chosen, the
thickness of the diode is utilized as effectively as possible for the
space charge zone, that is to say that the softness of a diode
dimensioned in this way is as good as that of a thicker diode without a
"deeper" field stop (that is to say a field stop zone introduced far into
the interior of the semiconductor volume), while its losses are equally
lower. In other words: with the deeply extending field stop doping, it is
possible to fabricate diodes with the same or improved softness with a
reduced thickness of the n-type base region.

[0031] FIGS. 3a-3c show the switching behavior of a 1200 V diode
fabricated according to the invention, with a rated current of 100A, in
comparison with a reference diode that does not have a proton field stop
situated deep in the semiconductor volume. Since the switching behavior
is more critical at a low bias current, a measurement was carried out
here at only 10% of the rated current and at the same time a relatively
high intermediate circuit voltage (800 V).

[0032] It can clearly be discerned that the reverse current 4 of the
reference device is chopped, while the reverse current of the component 5
according to the invention has a return with a moderate dI/dt (change in
current per change in time). A particularly good indicator of current
chopping is the gate voltage of the auxiliary switch, since it is
disturbed greatly by the current chopping of the diode: the gate voltage
of the reference (curve 6) oscillates to a significantly greater extent
than that of the diode according to the invention (curve 7). A possible
harmful influence on the adjacent components becomes particularly clear
here because the gate voltage, as a result of the oscillations of the
reference diode, momentarily even exceeds the switch-on threshold of the
auxiliary switch, but the latter in this case is too sluggish to react
immediately--and is possibly destroyed. The reference numerals 8 and 9
denote the substrate voltage profiles of the diode according to the
invention and the reference diode.

[0033] The proton doses of the individual implantations that are suitable
for realizing the proposed concept typically vary in the range of 0.5 . .
. 50*1013 protons/cm2, and the aggregate dose of all the
implantations typically varies in the range of 5 . . . 50*1013
protons/cm2. The heat treatments are intended to be carried out at
temperatures in the range of between 350° C. and 550° C.
over a few tens of minutes to a few hours, in which case a targeted
widening of the donor peaks realized can be realized as the temperature
budget increases, and the maximum dopant concentration in the
end-of-range region of the implantation is reduced, moreover, as the
temperature increases above approximately 400° C. At the same
time, the carrier lifetime is also increased in the region radiated
through, as a result of intensified annealing of the implantation
defects.

[0034] The highest dose, which serves for ensuring the blocking
capability, may preferably be implanted directly before the emitter. As a
result of the radiation damage of the preceding implantations with higher
energy, a lateral propagation of the hydrogen-induced donors is even to
be reckoned with here. Consequently, even particles that have masked the
shallow proton implantation can be underdiffused and, consequently,
reverse currents can be decreased or the yield in a reverse current test
can be increased.

[0035] In this case, the shallowest implantation may either directly
adjoin the emitter; however, it may also be spaced apart from the latter.
Thus, the depth of the implantation maximum of said shallowest
implantation may perfectly well be at a distance of up to approximately
one third of the base width of the chip from the rear-side emitter, in
order, between this doping peak and the rear-side emitter, to protect a
reservoir with higher charge carrier flooding against the field
punch-through.

[0036] According to the invention, then, in the case of a diode, a graded
field stop zone is produced, which enables a soft turn-off. What is
essential is that for this purpose at least two proton energies are
necessary and the integral dopant dose reaches the breakdown charge only
near to the cathodal end of the field stop zone.

[0037] The exemplary application of an IGBT shall now be discussed.
Firstly, the corresponding prior art will be considered.

[0038] The intention is to realize a field stop zone in IGBTs which, on
the one hand, guarantees a sufficient blocking capability of the
components, but on the other hand also enables satisfactory dynamic
properties--such as e.g. a sufficiently soft turn-off behavior and a high
short-circuit loading capacity. In particular, said field stop zone is
also intended to be realized at temperatures lying below 550° C.,
in order for the field stop zone not to be produced until in the largely
finished processed silicon wafer. This facilitates the use of relatively
thin silicon wafers, which entails a reduction of the overall losses in
the case of IGBTs having reverse voltages <1800 V.

[0039] Nowadays field stop zones are fabricated primarily by means of
implantation methods and subsequent diffusion steps, but the process
temperatures are relatively high. In the case of a phosphorus diffusion,
temperatures >1100° C. are required in order to produce a
sufficiently deep field stop zone with an economically tenable outlay.
Even in the case of a direct high-energy implantation into the
corresponding depth, temperatures of above 700° C. are still
required in order to anneal the radiation damage and to activate the
doping.

[0040] It has already been proposed (document U.S. Pat. No. 6,482,681 B1)
to fabricate such a stop zone by means of one or more proton irradiation
steps, energies lying between 100 and 500 keV being used in the
application of a plurality of proton irradiations. This is because proton
irradiations have the property of producing donors particularly in the
so-called "end-of-range" region, to be precise all the more donors the
higher the irradiation dose.

[0041] Corresponding experiments have revealed that, for simultaneously
realizing a soft turn-off behavior and a sufficient short-circuit
strength, a doping profile that leads out deeply is necessary for the
field stop zone, in a similar manner to that as can be realized e.g. with
a phosphorus diffusion at significantly higher temperatures in
combination with longer diffusion times.

[0042] Therefore, when using a proton irradiation for producing this
(preferably) n-doped field stop zone from the collector side, it is
absolutely necessary to use a multiple implantation in which the maximum
energy is at least 1 MeV. In this context, e.g. a triple (a) or quadruple
(b) proton irradiation with the following energy graduations would be
advantageous: [0043] a) 300 keV, 500 keV, 1 MeV; [0044] b) 300 keV, 500
keV, 1 MeV, 1.25 MeV.

[0043] This is because if the maximum energy of 500 keV is chosen, neither
the softness of the turn-off capability nor the required short-circuit
strength is provided.

[0044] Typical annealing temperatures for this irradiation lie in the
range of between 350 and 420° C. If, by contrast, the annealing
temperature is chosen in the range of between 420 and 550° C., the
(preferably) n-doped peak caused by the proton irradiation is widened
considerably, so that the number of irradiation steps may possibly be
reduced. A desirable side effect of this procedure may consist in the
raising of the carrier lifetime in the region radiated through as a
result of the increasing annealing of the irradiation-induced defects in
silicon, which lower the carrier lifetime.

[0045] It is also conceivable to realize the peak lying the deepest below
the collector-side surface by means of a proton irradiation from the
front side of the IGBT, preferably before the silicon wafer is brought to
its final thickness by thinning by grinding. The second deepest peak may
possibly also be realized in this way. A targeted widening of the peak or
peaks at temperatures lying between 400 and 550° C. is appropriate
here. Front-side processes whose permissible maximum temperature lies
below this annealing temperature can then be carried out after the proton
irradiation and this heat treatment. The irradiation energies required
for this are significantly higher, however, to be precise all the more
higher the thicker the silicon wafer, that is to say the higher the
required blocking capability of the components.

[0046] The irradiation doses should be chosen such that the deep-lying
peaks should be produced rather with a low dose, to be precise typically
in the range of between 1*1013 and 7*1013 protons/cm2,
while a high dose above approximately 5*1013 protons/cm2 should
be used in particular for producing the peak or the two peaks lying
closely below the wafer surface (with respect to the rear-side of the
device), in order to produce a sufficient number of donors so that the
breakdown charge is exceeded and the blocking capability of the
components is ensured. In this case, it is necessary to take account of
the fact that only a small percentage (approximately 1 to 2%) of the
implanted hydrogen dose is converted into donors.

[0047] The irradiation with a plurality of energies, primarily also higher
energies, has the advantage that, below particles which are usually
situated on the wafer surface during the proton irradiation, in any event
enough defects (induced by the irradiation) are also present which, in
combination with a lateral diffusion of the implanted hydrogen atoms also
in the region shaded by the undesirable particles, make available enough
donors for ensuring the blocking capability of the IGBT. This is because
both the irradiation-generated defects (in particular vacancies) and the
implanted hydrogen atoms are required for forming said donors. In order
to further safeguard the blocking capability, it is also possible to
effect an additional implantation of n-doping elements, such as e.g.
phosphorus, sulfur or selenium atoms, whose--albeit slight at the
temperatures used--lateral diffusion reliably precludes the negative
consequences of the shading effects described above.

[0048] According to the invention, therefore, a multiple implantation with
protons is carried out in the case of IGBTs, the irradiation energies
being chosen so as to produce a relatively deep doping profile (a doping
profile extending deep into the semiconductor profile) of the field stop
zone thus formed, which in turn leads to very good electrical properties
of the irradiated IGBTs. The highest implantation energy used should in
this case be at least 1 MeV, and the lowest should be a maximum of 500
keV.

[0049]FIG. 2 shows an example of a dopant concentration profile 3 of a
field stop layer of a semiconductor device according to the invention,
which can equally be used for a diode or an IGBT. A plurality of
maxima/minima can be seen, the height of the maxima increasing in the
direction toward the second connection region.

[0050]FIG. 4 shows a schematic diagram of the method for fabricating a
diode according to the invention. A diode 10 has a first connection
region 11 (preferably metal) and a second connection region 12
(preferably metal). A semiconductor volume 13 is arranged between the
first connection region 11 and a second connection region 12. The first
connection region 11 is adjoined by an anode region 15 (semiconductor
region) and the second connection region 12 is adjoined by a cathode
region 16 (semiconductor region).

[0051] Preferably, the thickness D1 of the field stop zone 14 is a maximum
of one third of the base width B1 of the semiconductor volume, the base
width being defined as the distance between the last pn junction 17,
before the field stop zone 14 (relative to a direction pointing from the
first connection region 11 to the second connection region 12) and the
interface 18 of the field stop zone 14 which faces the second connection
region 12. In order to produce a field stop zone 14 having the thickness
D1 within the semiconductor volume 13 having the thickness D2, protons
are radiated through the rear side of the diode 10, that is to say the
second connection region 12, using a plurality of implantation energies,
in which case, in principle, the first connection region 11 could also be
radiated through. As a result, the integral of the ionized dopant charge
over the semiconductor volume 13, proceeding from an interface 17 (pn
junction) of the anode region 15 that faces the second connection region
12, in the direction of the second connection region 12, produces a
quantity of charge corresponding to the breakdown charge of the
semiconductor device 10 only near the interface 18 of the field stop zone
14 which faces the second connection region 12.

[0052]FIG. 5 shows a schematic diagram of the method for fabricating an
IGBT according to the invention. An IGBT 20 has a first connection region
11 (preferably metal) and a second connection region 12 (preferably
metal). A semiconductor volume 13 is arranged between the first
connection region 11 and a second connection region 12. The first
connection region 11 is adjoined by a cell region 21 (semiconductor
region), and the second connection region 12 is adjoined by a rear-side
emitter region 22 (semiconductor region). The cell region 21 has, in a
known manner source regions 23, body regions 24, a gate 25 and an
insulation layer 26.

[0053] In order to produce a field stop zone 14 having the thickness D1
within the semiconductor volume 13 having the thickness D2, protons are
radiated through the rear side of the IGBT 20, that is to say the second
connection region 12, using a plurality of implantation energies, in
which case, in principle, the first connection region 11 could also be
radiated through. As a result, the integral of the ionized dopant charge
over the semiconductor volume 13, proceeding from an interface 27 (pn
junction) of the cell region 21 (to put it more precisely the body
regions 24) that faces the second connection region, in the direction of
the second connection region 12, produces a quantity of charge
corresponding to the breakdown charge of the semiconductor device 20 only
near the interface 28 of the field stop zone 14 which faces the second
connection region 12.

[0054] The penetration depths of the protons that are reached during
proton irradiation are related to one another such that the second
deepest penetration depth, compared with the deepest penetration depth,
has a distance of 30% to 60% of the value of the maximum penetration
depth. It is thus possible to obtain a particularly good softness during
the switching of the semiconductor device.